Ceramic dielectric material and method of making



Patented July 18, 1950 UNITED STATES CERAMIC DIELECTRIC MATERIAL AND L METHOD OF MAKING Ilouis Navias, Schenectady, N. Y. assignor to General Electric Company, a corporation of New York No Drawing. Application April 22, 1949. Serial No. 89,151

Iliis invention relates to ceramic dielectrics.

More particularly, it'relates to a ceramic dielectric formed from barium titanate and a fluoride of the group consistingof alkaline earth fluorides and lead fluoride, and to the process of manufacturing such ceramic dielectric material.

Ceramic dielectricsvpossessing high dielectric constant, suitably low power factor, high breakdown strength and favorable resistance properties are in demand for radio apparatus and other uses which require such favorable combination? of electrical characteristics. Among such applications are filter capacitors for the high voltage supply to television viewing tubes. In a typical case a capacitor for this use must have a capacity of several hundred micromicrofarads and withstand kilovolts D.-C. at an ambient temperature of 65 C. y

In the process of developing capacitors for such use, various methods of firing have been employed. According to one process the formed raw materials are placed on zirconia setting sand and fired, but the fired product exhibits low resistivity values. In addition the resistivity values are non-reproducible fromone sample to another prepared in exactly the same manner and from the same lots of raw ingredients. These erratic values of resistivity persist even though the dielectric constant, power factor and breakdown voltage values are favorable and persistent in nature from unit to unit.

It is an object of this invention to provide a ceramic dielectric material of superior resistivity.

It is a further object of the present invention to provide a ceramic dielectric material and method of manufacture wherein reproducible and favorlably high resistivities are attainable.

Other objects will become apparent from a consideration of the following description. 7,

It has been found that ceramic dielectrics of consistently reproducible and superior resistivity may be prepared by completely surrounding the material during firing with a neutral or non-reacting layer or body of zirconium dioxide sand or powder.

One prior method of firing ceramic dielectric materials has been to place the materials in a furnace on zirconia setting sand supported on a refractory plaque. This firing process produces materials having low dielectric constant, high power factor and low resistivity which detracts from their applicability to various uses. If the formed raw materials are placed in a covered reiractory sagger on a layer of zirconia sand while firing, the dielectric constant is raised and the 9 Claims. (01. 25-151) 2 power factor lowered. While the volume resistivity is raised somewhat, it is still not high enough and, like the other prior art'method values, it is not acceptably constant and reproducible from sample to sample.

It has now been found that if the formed raw materials such as, for example, a bariumtitanatestrontium fluoride mixture are completely embedded in or surrounded by granulated zirconia. the volume resistivity increases sharply, and, most surprisingly, the higher resistivity values 7 are reproducible without special precautions in trends in the values of the various factors complementing the favorable resistivity characteristic. Thus the advantages of the present procedure are attained without sacrifice of other desirable qualities.

While the exact mechanism by which completely embedding the formed or shaped barium titanate-fluoride firing mixture in zirconia sand produces such good results is not known, it is believed that the fluoride compound in this way during dissociation and gasification tends to maintain a beneficial atmosphere around the body being fired. It is known that the fluoride acts as a flux in producing a compact fired material and also that little or no fluoride is present in the finished article.

While the present invention is described with relation to the use of strontium fluoride in conjunction with the barium titanate, any fluoride of the alkaline earth group, including calcium, barium, strontium and magnesium, is suitable, as well as lead fluoride. Likewise, the fluoride may be added in any amount desired up to about 10%; amounts over this limit do not add to the desirable qualities achieved with smaller amounts.

The usual fabricating and firing techniques are used in the manufacture of the improved dielectric material of the present invention except, of course, that the raw pressed materialis completely embedded in or surrounded by zirconia sand during firing. The following will serve as an example of the process.

A mixture of barium titanate and 5% strontium fluoride by weight is blended or mixed with water to produce a rather thick slurry which permits sufilcient movement of the slip volume under the impulse of the mixing blades of, for example, a colloid mill and yet holds the particles bon/ tetrachloride or a polyvinyl alcohol-water sol tion, among others, may be used if desired. Pfecaution should be taken that the binder is completely burned out before the peak firing temperature isreached.

The compacted raw dielectric elements or units are placed in a cool furnace and completely embedded in and. surrounded by electrically fused zirconia sand. A layer at least one-half inch thick on all sides of the elements has been found very suitable, greater thicknesses neither adding to nor detracting from the results. The dielectric and embedding materials are preferably placed on refractory plaques or saggers. The rate of temperature increase for firing is varied from 200 C. to 300 C. per hour up to the peak temperature of about 1350 C. which is maintained for about three hours. The elements are allowed to cool in the furnace nearly to room temperature before removal.

The fired material, which is typically in the form of discs,,may be made into capacitor elements in the usual way. Thus a silver paste or paint may be applied to each face of the disc, given the proper heat treatment to obtain a bonded metal surface and leads such as rivets soldered thereto. The unit may, if desired, then be encased in insulating material.

A number of discs approximately 0.3 inch What I claim as new and desire to secure by Letters Patent of the United'States is:

1. In the manufacture of ceramic dielectric elements from barium titanate and a fluoride of the group consisting of alkaline earth fluorides and lead fluoride, the process which includes blending the raw materials with water, dehyrating and powdering the mixture, forming the powdered material into the desired'shap under a pressure of not less than two tom: per square inch, completely embedding the formed material in powdered zirconia and firing it at an elevated temperature up to 1350 C.

2. In the manufacture of ceramic dielectric elements containing not less than 90% by weight barium titanate and not more than 10% by weight of a fluoride of the group consisting of alkaline earth fluorides and lead fluoride, the process which comprises mixing the barium titanate and the fluoride with water, dehydrating and reducing the mixture to a powder, forming the powder into the desired shape under a pressure of at least two tons per square inch, completely embedding the shaped material in zirconium dioxide sand and firing at 1350 C. for about three hours.

3. In the manufacture of ceramic dielectric elements from barium titanate and a fluoride of the group consisting of alkaline earth fluorides 0 and lead fluoride, the process which comprises mixing the raw materials with water, dehydrata temperature of about 1350 C. for about three hours and cooling to room temperature.

4. In the manufacture of ceramic dielectric units from not less than 90% by weight barium thick and one inch in diameter containing 5 titanate and not more than 10% by weight stronbarium titanate and 5% strontium fluoride were tillm flu ride, the process which comp s flred for 3 hours at 5 by the prior n; and ing the barium titanate and strontium fluoride present methods and tested with the following with e y in d cin he mixresults, the D.-C. voltage being applied for tenture to a powder, forming the powder into the minutes to attain equilibrium conditions and the desired shape under a pressure of at least two volume resistivity then being measured at 65 C. tons per square inch, completely surrounding The dielectric constant and power factor were the shaped material with zirconium oxide sand measured at room temperature. and firing at 1350 C. for about three hours.

D.- new Dielectric Power figs 0 1 2 0. Placing of Disks m 3%,; down in Cycles Cycles E Kv. m: 10 Resistivity, Ohm- Minutes 0111x10 0n layer of ZrOr sand on refractory as: 3-5222 s: as: 0 la 12 0 d tainedin I a; as 2;: I as.

4,000 0.0214 00.5 1.2 use 3,055 0. 020s 43.1 1.2 (140 Embedded in zro, sand in a refractory 4,110 0.0211 4.5.1 1.2 0.10

segger 3,!30 0. 0224 55.4 7.2 1.25

The capacitance of all of the above elements 5. In the manufacture of ceramic dielectric was about 350 micromicrofarads under the test conditions.

Dielectric elements prepared according to the present invention are long-lived, six units having been held at 15 kv. D.-C. at room temperature for 200 hours and then at 65 C. at 15 kv. D.-C. for another 200 hours without signs of failure. Another group of four units was subjected to 18 kv. D.-C. at room temperature for 250 hours with no indication of breakdown.

elements from by weight barium titanate and 5% by weight strontium fluoride, the process which includes mixing the barium titanate and strontium fluoride with water to form a blend,

dehydrating and powdering the blend, forming the powdered material into the desired shape under a pressure of not less than two tons'per square inch, completely embedding the formed material in powdered zirconia and firing for about three hours at 1350 C.

6. In the manufacture of ceramic dielectric units from barium titanate and a fluoride of the group consisting of alkaline earth fluorides and lead fluoride, the improvement which comprises firing up to a temperature of 1350 C. the units while embedded in zirconium dioxide sand.

'7. The process of making ceramic dielectric elements of barium titanate and a fluoride of the group consisting of alkaline earth fluorides and lead fluoride which includes firing up to a temperature of 1350 C. the elements while completely surrounded by zirconium dioxide.

8. In the manufacture of ceramic dielectric elements from barium titanate and strontium fluoride, the step which comprises firing up to a temperature of 1350 C. the formed raw materials while embedded in zirconium dioxide sand.

9. The ceramic dielectric unit made by the 5 has been fired in the open air.

process of claim 1, said product having a higher Volume resistivity, a, higher dielectric constant, a lower power factor, and a higher breakdown strength than the corresponding product which LOUIS NAVIAS.

REFERENCES CITED The following references are of record in the 10 file of this patent:

UNITED STATES PATENTS 

1. IN THE MANUFACTURE OF CERAMIC DIELECTRIC ELEMENTS FROM BARIUM TITANATE AND A FLUORIDE OF THE GROUP CONSISTING OF ALKALINE EARTH FLUORIDES AND LEAD FLUORIDE, THE PROCESS WHICH INCLUDES BLENDING THE RAW MATERIALS WITH WATER, DEHYDRATING AND POWDERING THE MIXTURE, FORMING THE POWDERED MATERIAL INTO THE DESIRED SHAPE UNDER A PRESSURE OF NOT LESS THAN TWO TONS PER SQUARE INCH, COMPLETELY EMBEDDING THE FORMED MATERIAL IN POWDERED ZIRCONIA AND FIRING IT AT AN ELEVATED TEMPERATURE UP TO 1350*C. 